The prior art includes systems for relaying optical information between two beacons. This is conventionally accomplished by first detecting and demodulating the optical information received by the first beacon from an optical source, subsequently synthesizing a optical beam by modulating another optical source with this information, and, finally, directing the new optical beam to the second beacon. This multi-element repeater system has application to well-defined relay modules, along optical fiber links for example, or for N×M interconnects for photonic networks, among others. However, in the general case, where propagation errors may be dynamic, and where the incident beams can arrive over a large field-of-view, a more robust interconnection system is required. These problems and limitations are addressed by this invention.
The prior art also includes systems comprising a set of tilt-mirror compensators which are used for correcting certain errors. Such systems can only compensate for the lowest-order errors such as tilt and astigmatism errors. Other low-order errors, such as focus, can be corrected with a variable focus element. However, these systems are unable to compensate for higher order propagation errors such as general wavefront distortions due to propagation through turbulent atmospheres, multi-mode optical fibers, etc. Thus, a system and method are needed that provide ways of compensating for these errors.
The present invention addresses the general case of phase (wavefront) errors. In this connection, the prior art includes the Double-pumped Phase Conjugate Mirror (DPCM). The DPCM does not require any servo-loop devices, since it proceeds via an all-optical nonlinear interaction. However, the DPCM requires the power carried by the incident laser beam to be above a given threshold, in order to properly function. This threshold generally ranges between a few μW/cm2 to a few mW/cm2, depending on the particular crystal used for the DPCM. Some examples of adequate crystals include BaTiO3 and InP. Moreover, the response time of a DPCM depends on the intensity of the incident beams, and the intensities of the two incident beams need to have similar values for the device to function optimally (fast response time, stable operation, and suitable wavefront compensation). Finally, the DPCM is lossy and the insertion loss can be large, approaching 30% or more.
In contrast, the present device has a very low insertion loss (it only requires enough light to be sensed by the wavefront error sensor), and can function with input beams with intensities which need not to be equal (i.e., not necessarily balanced). Similar to conventional adaptive optical systems, the wavefront compensation capability will be a function of the number of equivalent pixels, or phase actuators, relative to the number of resolvable coherent phase patches which need to be phased up or corrected.
One object of the invention is to provide a system and method for relaying optical information from one transceiver to another. Specifically, this invention will direct a first optical beam emanating from a first transceiver and travelling to a second transceiver, into the reverse direction of a second optical beam emanating from the second transceiver and traveling to the first transceiver. The beams can be encoded, so that a communication link is realized with diffraction-limited capability. In its most basic form, a simple pair of tilt mirrors may be used to direct one beam into the reverse direction of the other. However, in general, the beams are not plane waves, and may have undergone time-varying (i.e., dynamically varying) propagation distortions, including atmospheric distortions, multi-mode fiber-induced distortion, etc. Therefore, an adaptive optical element is preferably used to compensate for, and to track out, these undesirable time-varying distortions, and, at the same time, provide a means for coupling the light from one direction into the other, and vice versa (without loss of the desired modulation). Since this system provides for coupling of the two optical beams, no local detector or source is required at the location of the interconnect module. Moreover, the optical beams that leave the interconnect module propagate in precisely the reverse direction of the incident beams (i.e., they are mutually phase-conjugate replicas of the incident beams). Thus, pointing and tracking is realized with this system, so that the system performance is not compromised (i.e., low insertion loss and high directivity). Finally, modulation is preserved on the various beams, so that information can be transferred from one station to another station, with diffraction-limited performance, and subject to typical adaptive optical design issues and constraints, such as diffraction, dispersion, depolarization, the compensation bandwidth, the spatial bandwidth of the system (e.g., the number of resolvable pixels for wavefront reconstruction), etc.
Applications of the present invention include optical “relay nodes” for free-space, space-based or terrestrial-based, as well as for guided-wave based (e.g., coupling of the output of a single or multi-mode fiber to another fiber or to a free-space path), optical communication and image relay links, or combinations thereof. Many applications do not afford the luxury of line-of-sight viewing between the stations at the end points of the communication link. For example, a mountain may obstruct the end points for direct viewing, or two satellites may no longer “see” each other. To overcome this problem, an intermediate “relay node” or interconnection system is required, which may be placed on a hilltop or on an intermediate satellite, such that the interconnection system is in the line of sight of both stations. It may also be necessary to optically relay (one-way or two-way) information from one subsystem (e.g., a multimode fiber) into another subsystem (e.g., an array of optical modulators, detectors, etc.).
As shown in FIG. 1, the prior art discloses a method to first detect and demodulate the beam (originating from a first station) at the mid-point (e.g., hill-top) of the link between two stations, then to encode this information onto another laser, and finally direct the encoded data to a second station to complete the link (on the other side of the hill-top). This approach, however, does not compensate for propagation distortions.
The present invention provides for an automatic re-directing of the beam, as it arrives at the hill-top, to the second half of the link, as shown in FIG. 2. Moreover, the invention compensates for propagation distortions, so that the beam will arrive at the other end of the link without distortion. This invention enables such an intermediate node to be realized, without the usual photonic repeater requirements of high-bandwidth photo-detection, modulation and retransmission of the data. In this invention, the temporal modulation format imposed onto the beam from its initial point of origin is preserved. As it goes through the interconnection system only its wavefronts are modified, while its temporal encoding is maintained. Further, the system can function using mutually incoherent sources (e.g., free-running lasers at each end point of the link). When both of these lasers impinge onto the system, the beam from one of the end-points will be directed into the wavefront-reversed direction of the path that the second beam took, thereby “finding” and arriving to the other end of the link distortion-free (assuming usual time scale of beam formation by the system, range, atmosphere distortion time scale, and motion of the source locations during the optical transit time).
Additionally, the system of the present invention provides for “auto-tracking”. Indeed, if the end-point stations are moving, the interconnect can track or follow the moving stations. This assumes that the stations move slowly with respect to the reconfiguration time of the interconnect and the time/spacial scale of the dynamic distortions. The system provides for propagation-distortion compensation as well.
A related application is in the area of space-based low-cost relay mirrors. A pair of large-area telescopes are used to collect a weak signal, and then relay the beam to another location. These lightweight mirrors, which may be made of thin membranes (mylar, etc.), often possess optical distortions because the lightweight material they are made of can easily deform. The system performance is thereby degraded. By placing the proposed invention between the pair of large-area relay mirrors, the local mirror aberrations, as well as path distortions experienced by the two incident beams, can be corrected in real-time. Other potential areas of application include stratospheric relay platforms, such as LEO and MEO satellites and other airborne systems, with application to backbone feeder lines, as well as dynamic links for optical fibers, laser sources and beam combining systems. In the latter case, a given incident probe beam can be used as a local reference beam, which can, as a result of the interconnection system, phase up a collection of single-frequency, but randomly phased oscillators, including optical fiber amplifiers and oscillators.